A genetic approach, comparing prototype and reassortant La Crosse (LAC) and snowshoe hare (SSH) viruses was used to define at least partially the molecular basis for dissemination of bunyaviruses from midgut cells and subsequent oral transmission by Aedes triseriatus mosquitoes. The middle-sized RNA segment through its gene products the glycoproteins, appears to be the major determinant of these phenomena. Reassortment of bunyaviruses was demonstrated in dually infected Aedes triseriatus. After infection with either homologous or heterologous mixtures of group I and II temperature sensitive mutants of SSH and/or LAC viruses, both wild type and ts mutants were obtained from the mosquitoes. In one heterologous cross, one of the viruses was demonstrated to contain two RNA segments from the SSH parent and one segment from the LAC parent--a new reassortant virus. Transmission of reassortant viruses to mice was also demonstrated, and one of the viruses was shown to be a reassortant virus by oligonucleotide fingerprinting.